Method for detecting oscillation criticality of servo control system

ABSTRACT

Control parameters are regulated without oscillating a servo control system. Respective control parameters P[p] (p=0, 1 - - - ), arranged in the order of gradually increasing a chance of oscillating the servo control system, are set sequentially starting with P[ 0 ], a torque command Tr or a fluctuation amount σ, showing a variation in frequency component of vibration of a speed feedback amount ω, is determined for each of control parameters, and the servo control system is assumed to have reached an oscillation criticality when a fluctuation amount exceeds a specified value to thereby reset a control parameter set at that time to P[p−q] (q; natural number) or to P[ 0 ].

TECHNICAL FIELD

The present invention relates to a method for detecting the oscillationcriticality of a servo control system and adjusting control parameters.

BACKGROUND ART

Usually, in order to control an object to be controlled in a servocontrol system, feedback control is carried out, by which an amount ofoperation with respect to the object to be controlled is acquired from adeviation between a command issued from an upper-level apparatus and anactual amount of control. FIG. 5 is a block diagram showing aconstruction of a servo control system by which speed control is carriedout. The servo control system is composed of a subtractor 1, a speedcontroller 2, a torque amplifier 3, a servo motor (M) 4, an encoder (E)5, a machine 6, and a differentiator 7. The speed controller 2 is ameans for controlling the machine 6, which is an object to becontrolled, and is a proportion-integration-differentiation unit(hereinafter called a “PID controller”). Herein, Kv, Ki and Kd arecontrol parameters of the speed controller 2. Kv is a proportional gain,Ki is a reciprocal number of an integration time constant, and Kd is adifferentiation time.

The subtractor 1 subtracts a speed feedback ω from a speed command ωrinputted from an upper-level apparatus (not illustrated), and outputs aspeed deviation. The speed controller 2 carries out PID control byinputting the speed deviation and outputs a torque command Tr. Thetorque amplifier 3 outputs a current to the servomotor 4 by inputtingthe torque command Tr. The servomotor 4 rotates by the current and themachine 6 moves by the rotation movement. The encoder 5 is attached tothe servomotor 4 and outputs the rotation position of the servomotor 4.The differentiator 7 differentiates the rotation position, which isoutputted from the encoder 5, and outputs a speed feedback ω. Also, inthe case where the above-described servo control system is a digitalcontrol system that carries out control per sampling cycle, usually,there are many cases where, using a difference detector instead of thedifferentiator 7, a difference between the previous rotation positionand this rotation position is made into the speed feedback ω.

FIG. 6 is an equivalence block diagram of the servo control system shownin FIG. 5. In FIG. 6, a description is based on the assumption that themachine 6 is completely rigid, the response of the torque amplifier 3 isideal for simplifying the description thereof, and the speed controller2 carries out proportional control on the basis of only the proportionalgain Kv. FIG. 6(a) is an equivalence block diagram of a servo controlsystem where an inertia of the machine 6 is assumed to be J, and FIG.6(b) is an equivalence block diagram of a servo control system where aninertia of the machine 6 is assumed to be 2J. Herein, it is also assumedthat values of the proportional gains Kv in FIGS. 6(a) and (b) are thesame.

FIG. 7 is a graph showing a transition response of the speed feedback ωwith respect to a step-like speed command ωr in FIGS. 6(a) and (b). Asshown in FIG. 7, where the inertia of the machine 6 is changed from J to2J, it is understood that the response of the servo control systemchanges and the followability of the servo control system is worsened.

Therefore, in such a servo control system, in a case where parameters ofan object to be controlled such as the inertia of the machine 6 ischanged, it is necessary to vary the control parameters such as theproportional gain Kv of the speed controller 2 in response to the valueof the inertia so that the machine 6 is optimally controlled. However,if the control parameters such as the proportional gain Kv arethoughtlessly changed, there may be a cause a concern that oscillationsoccur due to resonance of the mechanical system including the machine 6and useless time of the servo control system, etc. Generally, the largerthe proportional gain Kv becomes, the greater the followability to thespeed command ωr is increased. But, if the proportional gain Kv isincreased too much, the servo control system is likely to oscillate.

It is assumed that, among the values of the proportional gains Kv, anarea of values of the proportional gain Kv is defined to be Area “a”when the servo control system does not oscillate and is in a stablestate, an area of values of the proportional gain Kv is defined to beArea “b” when the servo control system is in oscillation criticality,and an area of values of the proportional gain Kv is defined to be Area“c” when the servo control system is in a completely oscillating state.FIG. 8 are graphs showing a frequency response G(f) of the speedfeedback ω in terms of a logarithm when the values of the proportionalgain Kv are in respective areas.

FIG. 8(a) shows a state of log G(f) where the values of the proportionalgain Kv are in Area “a”, wherein log G(f) has a small peak in thevicinity of f=0, and the value of log G(f) is totally low. FIG. 8(b)shows a state of log G(f) where the values of the proportional gain Kvare in Area “b”, wherein, although log G(f) is distributed in a widefrequency band, the peak thereof is not so high. FIG. 8(c) shows a stateof a frequency response log G(f) where the values of the proportionalgain Kv are in Area “c”, wherein log G(f) has a very high peak at acertain frequency band. It is understood that the servo control systemoscillates in this frequency band. In addition, the frequency responseof the torque command Tr shows a tendency similar to the frequencyresponse of the above-described speed feedback ω.

As described above, the control parameters such as the proportional gainKv causes the followability of the servo control system to be worsenedif the values thereof are small, and brings about oscillations in theservo control system if the values thereof are large. Therefore, it isrecommended that the control parameters such as the proportional gain Kvare set to optimal values.

As a method for optimally obtaining control parameters such as theproportional gain Kv, etc., such a method is disclosed by JapanesePatent Publication No. 2861394, in which an amplitude and a frequency offluctuations of the speed feedback ω in an appointed duration of timeare calculated, and the control parameters are adjusted by judging that,if the amplitude value and frequency value exceed appointed values,oscillations have occurred. However, with the method disclosed by theabove-described patent publication, the control parameters cannot beadjusted unless actual oscillation occurs. For this reason, where thismethod is used, actual oscillation occurs before commencing to adjustthe control parameters, whereby such problems are brought about, bywhich the machine 6 connected to the servomotor 4 may be damaged due toinfluences of the oscillation, or large noise may be generated.

On the other hand, it is experimentally made clear that shaking of thespeed feedback ω and torque command Tr changes in response tofluctuations in the proportional gain Kv. The shaking of the speedfeedback ω and torque command Tr means unevenness of frequencycomponents in oscillations of the speed feedback ω and torque commandTr. FIG. 9 is a graph showing the relationship between the proportionalgain Kv and the shaking amount of the speed feedback ω. Where the valueof the proportional gain Kv is in Area “a”, the shaking amount of thespeed feedback ω is small. And where the value of the proportional gainKv is in Area “b”, the shaking amount of the speed feedback ω isgradually increased in line with an increase in the value of theproportional gain Kv. In Area “c”, that is, the oscillation area,although the speed feedback ω consistently oscillates, the frequencycomponents of the oscillations are made almost constant, wherein theshaking amount is made small. Such a tendency is also brought about withrespect to the shaking amount of the torque command Tr.

As described above, conventionally, when adjusting the controlparameters in the servo control system, the amplitude and frequency offluctuations of the speed feedback in an appointed duration of time arecalculated, wherein if the amplitude and frequency exceed appointedvalues, it is judged that the servo control system oscillates, and thecontrol parameters are adjusted. However, with this method, the controlparameters cannot be adjusted unless actual oscillations begin.Accordingly, where the method is used, actual oscillations occur beforecommencing to adjust the control parameters, wherein there occur suchproblems, by which a machine connected to the servomotor is damaged dueto influences of the oscillations, and large noise is generated.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor detecting oscillation criticality of a servo control system that canadjust control parameters without causing the servo control system tooscillate.

In order to solve the above-described problem, the present invention isa method for detecting oscillation criticality of a servo controlsystem, which can adjust control parameters of means for controlling anobject to be controlled, by carrying out, per sampling cycle, input of adeviation between the amount of control fed back from a servomotor todrive the above-described object to be controlled and a command valueinputted from an upper-level apparatus, and output of a torque commandto a torque amplifier that outputs a current to the servomotor,comprising the steps of:

stepwise changing values of control parameters, and measuring a shakingamount being uneven in a frequency component of oscillations of theabove-described variable per value of the control parameters;

judging that, when the above-described shaking amount exceeds anappointed value, the servo control system reaches oscillationcriticality; and

adjusting the above-described control parameters by returning the valuesof the control parameters, which are set in the above-describedcontrolling means, by only appointed steps.

The method for detecting oscillation criticality of a servo controlsystem according to the invention can detect values of controlparameters set when the servo control system reaches the oscillationcriticality by detecting the shaking amount, that is, unevenness in afrequency component of oscillations of a variable that is maximized whenthe servo control system is in oscillation criticality, whereby thecontrol parameters can be adjusted without causing the servo controlsystem to oscillate.

As described above, the method for detecting oscillation criticality ofa servo control system according to the invention can detect controlparameters, when the servo control system is in oscillation criticality,by obtaining the shaking amount, that is, unevenness in frequency ofoscillations of a torque command or speed feedback, which is maximizedin the oscillation criticality area of the servo control system, wherebythe control parameters can be adjusted without causing the servo controlsystem to oscillate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 are perspective views showing a construction of a servo controlsystem in a method for detecting oscillation criticality of the servocontrol system according to one embodiment of the invention;

FIG. 2 is a flowchart showing a method for detecting oscillationcriticality of a servo control system according to the embodiment of theinvention;

FIG. 3 is a flowchart showing movements when obtaining a shaking amountin the method for detecting oscillation criticality of the servo controlsystem according to the embodiment of the invention;

FIG. 4 is a graph showing an inverted state of signs of a differencebetween the present torque command and the previous torque command withrespect to fluctuations in the torque command according to the methodfor detecting oscillation criticality of a servo control systemaccording to the embodiment of the invention;

FIG. 5 is a block diagram showing a construction of a servo controlsystem that carries out speed control;

FIG. 6 are equivalence block diagrams of the servo control system ofFIG. 5;

FIG. 7 is a graph showing the transition response of a speed feedback ωwith respect to a step-like speed command ωr;

FIG. 8 are graphs showing frequency response log G(f) of the speedfeedback ω; and

FIG. 9 is a graph showing the relationship between a proportional gainKv and shaking of the speed feedback ω.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a detailed description is given of a method for detectingoscillation criticality of a servo control system according to oneembodiment of the invention with reference to the accompanying drawings.In all the drawings, components that are given the same referencenumbers are identical to each other.

The method for detecting oscillation criticality of a servo controlsystem according to the present embodiment is a method in which thefocus is on the relationship between control parameters such as aproportional gain Kv, a reciprocal Ki of integration time constant, anddifferential time Kd, which are shown in FIG. 9, and shaking, that is,unevenness of frequency components of oscillation of a torque command Trand speed feedback ω. In the method for detecting oscillationcriticality of a servo control system according to the embodiment,values of the control parameters are stepwise increased, and at the sametime, the shaking amount of the torque command Tr and speed feedback ωis measured in the values of the control parameters, wherein where theshaking amount exceeds an appointed amount, it is judged that the servocontrol system reaches oscillation criticality, and the controlparameters are adjusted by returning the values of the controlparameters to be set, by an appointed step toward the original values.

As shown in FIGS. 1(a) and (b), in the method for detecting oscillationcriticality of a servo control system according to the presentinvention, a personal computer 13 or a teaching pendant 14 is connectedto a servo control apparatus 11. The personal computer 13 or teachingpendant 14 is used when inputting control parameters into the servocontrol apparatus 11, and displays the results of adjustment, etc., ofthe control parameters. Hereinafter, to simplify the description, thepersonal computer 13 or teaching pendant 14 is merely called an“input/output unit”. Also, the servo control apparatus 11 is providedwith a speed controller 2 and a torque amplifier 3, which are shown inFIG. 5. The control parameters of the speed controller 2 are adjustedwhile inputting a speed command ω from an upper-level apparatus 12.

In the method for detecting oscillation criticality of a servo controlsystem according to the embodiment, several values of control parametersare prepared in advance. These are called P [0], P [1], P [2], . . . .These control parameters are arranged in such an order that it becomesdifficult for the servo control system to oscillate. For example, thecontrol parameters are P[0]<P[1]<P[2], . . .

FIG. 2 is a flowchart showing the method for detecting oscillationcriticality of a servo control system according to the presentembodiment. Herein, reference letter p denotes an index value of acontrol parameter that is currently set, and reference letter q denotesa value, which is a natural number more than 1 and is defined inadvance. In the method for detecting oscillation criticality of a servocontrol system according to the present embodiment, where it is judgedthat oscillation criticality occurs when P[p] is set, P[p-q] that is qsteps before P[p] is set in the servo control apparatus 11 as a controlparameter.

First, 1 is set in p (Step S802). Next, the control parameter P[0] isinputted from the input/output unit into the servo control apparatus 11(Step S803). Next, a control parameter P[p] is inputted from theinput/output unit into the servo control apparatus 11 (Step S804), and acontrol parameter P[p] is set in the servo control apparatus 11(Initially, since p=1 is set, P[1] is set) (Step S805). And, p isincremented (Step S806). And, it is judged whether or not the servocontrol system is in oscillation criticality (Step S807). Where it isjudged that the servo control system is not in oscillation criticality,it is judged whether or not a movement of the servo control system atthe set control parameter meets a required control capacity, wherein ifnot satisfactory, the process returns to Step S804, and if satisfactory,the process ends.

In addition, in Step S807, where the servo control system is inoscillation criticality, it is judged whether or not p is larger than q(Step S808). Where p is larger than q, P[p-q] is set in the servocontrol apparatus 11 as the optimal parameter control (Step S809), andwhere p is smaller than q, P[0] is finally set in the servo controlapparatus 11 as an optimal control parameter (Step S810). And, it isdisplayed in the input/output unit that oscillation criticality has beenreached, and the control parameter has been changed (Step S811), and itis further displayed whether or not re-adjustment is carried out (StepS812). Where the re-adjustment is carried out, the process returns toStep S802, and where the re-adjustment is not carried out, the processends.

In Step S807, criticality judgment is carried out by obtaining theshaking amount of the torque command Tr. In the method for detectingoscillation criticality of a servo control system according to theinvention, first, a torque command value Tr[i] is sampled once everysampling cycle Ts, the number N[m] of times of inverting the sign iscalculated, which is the number of times for which the sign of adifference obtained by subtracting the torque command value Tr[i−1] fromthe torque command value Tr[i] during an appointed number I₀ of samplingtimes is inverted. And, the calculation of the number N[m] of times ofinverting the sign is carried out by an appointed number M₀ of times,and the standard deviation value σ of the plural number N[m] of times ofinverting the signs obtained is calculated, wherein the standarddeviation value σ is made into the shaking amount σ at the controlparameter, wherein it is assumed that O≦i<I₀ and O≦m<M₀ are set.

FIG. 3 is a flowchart showing movements when obtaining the shakingamount σ in the method for detecting oscillation criticality of a servocontrol system according to the invention. In the method for detectingoscillation criticality of servo control system according to theembodiment, first, i and m are initialized (Step S101). And, m isincremented, and Nm is initialized (Step S102). A sampling time i isincremented (Step S103). Next, this time torque command value Tr[i]isacquired (Step S104). And, on the basis of the previous torque commandvalue Tr[i−1] and the present torque command value Tr[i], a calculationis carried out with respect to the following expression (Step S105).

Xi=Sign(Tr[i]−Tr[i−1])  (1)

Sign ( ) is a function that returns 1 where the sign of a figure in thebracket ( ) is positive and returns −1 where the sign thereof isnegative. Next, on the basis of the product obtained from the presentcalculated X[i] and the previous calculated X[i−1], it is judged whetheror not the signal of X[i] is inverted (Step S106). In Step S106, wherethe signal of X[i] is inverted, the number N[m] of times of invertingthe sign is incremented (Step S107). In Step 106, where the sign of X[i]is not inverted, the incrementing of the number N[m] of times ofinverting the sign is not carried out.

Next, it is judged whether or not i is larger than an appointed value I₀(Step S108). If the i is smaller than the appointed value I₀, theprocess returns to Step S102. Where the i is larger than the appointedvalue I₀ in Step S108, it is judged whether or not m is larger than anappointed value M₀ (Step S109). If the m is smaller than the appointedvalue M₀, the process returns to step S102. If the m is larger than theappointed value M₀, the shaking amount σ is obtained by the followingexpression 1.

$\begin{matrix}{\sigma = \sqrt{( {1/{Mo}} ){\sum\limits_{j - 0}^{{Mo} - 1}( {{N\lbrack j\rbrack} - {\langle N\rangle}} )^{2}}}} & {{Expression}\quad 1}\end{matrix}$

where <N> is the average value (O≦m<M₀) of N[m]

For example, a variation in the torque command Tr is as shown in FIG. 4where M₀=3, the number N[m] of times of inverting the sign with respectto a difference in the torque command at m=0 through 2 becomes 5, 4 and9, wherein the average value <N> of N[m] becomes (5+4+9)/3=6. Then, theshaking amount σ can be obtained as shown below.

σ=[{(5−6)²+(4−6)²+(9−6)²}/3]^(1/2)=2.16

Also, appointed time TO in FIG. 4 is a product obtained by multiplyingthe number I₀ of sampling times by the sampling cycle Ts.

σ in Expression 1 is the standard deviation of the number N[m] (m=Othrough M⁰⁻¹) of times of inverting the sign. The standard deviation isin connection with the frequency of the torque command Tr. The largerthe frequency of the torque command Tr becomes, the larger the value ofN[m] becomes, and the smaller the frequency of the torque command Trbecomes, the smaller the value of N[m] becomes. Therefore, the standarddeviation of N[m] becomes one of the indexes expressing the unevennessof the frequency of the torque command Tr.

Next, the shaking amount σ obtained in Step S110 is compared with theappointed value σ₀ (Step S111). Where the shaking amount σ exceeds theappointed amount σ₀, it is judged that the servo control system is inoscillation criticality (Step S112), and where the shaking amount σ isless than the appointed shaking amount σ₀, it is judged that the servocontrol system is not in oscillation criticality (Step S113). Then, theprocess ends.

Further, in the method for detecting oscillation criticality of a servocontrol system according to the embodiment, the oscillation criticalityis detected by obtaining the shaking amount σ of the torque command Tr.However, the oscillation criticality may be detected by obtaining theshaking amount of the speed feedback ω.

As described above, in the method for detecting oscillation criticalityof a servo control system according to the embodiment, since, byobtaining the shaking amount of a frequency of oscillation of the torquecommand or the speed feedback, which is maximized when the mechanicalsystem is in oscillation criticality, it is possible to adjust thecontrol parameters without setting the control parameters being in theoscillation area in the servo control system 11, the control parameterscan be adjusted without causing the servo control system to oscillate.

Also, in the method for detecting oscillation criticality of a servocontrol system according to the embodiment, the standard deviation valueof the number N[m] of times of inverting the sign of a differencebetween the present torque command Tr[i] and the previous torque commandTr[i−1] is made into the shaking amount σ. The method is one methodsuitable for being mounted in a servo control system because thecalculation is simple. However, various methods such as a high-speedFourier transformation method (FFT), etc., are available in addition tothe method for obtaining a shaking amount of a torque command Tr andspeed feedback ω. The method for detecting oscillation criticality of aservo control system according to the embodiment does not regulate anyof the methods for obtaining the shaking amount according to theinvention. Also, the method for detecting oscillation criticality of aservo control system according to the embodiment may be applicable tonot only a servo control system for which speed control is carried out,but also a servo control system for which position control is carriedout.

The method for detecting oscillation criticality of a servo controlsystem according to the embodiment is such that the control parametersare adjusted before the servo control system commences to operate.However, there are cases where a servo control system oscillates due togradual changes of mechanical conditions during its operation. Themethod for detecting oscillation criticality of a servo control systemaccording to the embodiment is easily applicable to real-time adjustmentof control parameters, which is carried out when control conditions suchas mechanical conditions change during the operation of a servo controlsystem. For example, the servo control apparatus 11 is actuated as shownin the flowchart of FIG. 3 even during the operation of a servo controlsystem, so that the shaking amount σ of a torque command Tr is obtained,and, where the shaking amount σ exceeds an appointed amount σ₀, theservo control apparatus 11 is actuated so that the value of the controlparameter is changed to a value, which is changed by an appointed valuefrom the present set value, judging that the servo control system is inoscillation criticality. In this case, the servo control apparatus 11may obtain the shaking amount σ of the torque command Tr as shown in theflowchart of FIG. 3, and may obtain the shaking amount of the speedfeedback, which is a value of control of the servo control system.

INDUSTRIAL APPLICABILITY

Where the method for detecting oscillation criticality of a servocontrol system according to the present invention is used, since it ispossible to detect control parameters when the servo control system isin oscillation criticality, by obtaining the shaking amount when theoscillation frequency of a torque command or speed feedback fluctuate,which is maximized in the oscillation criticality area of the servocontrol system, such an effect can be brought about, by which thecontrol parameters can be adjusted without causing the servo controlsystem to oscillate.

What is claimed is:
 1. A method for detecting oscillation criticality ofa servo control system, comprising: stepwise changing values of at leastone control parameter, measuring a shaking amount of the controlparameter, where the shaking amount is an unevenness in a frequencycomponent of oscillations of said control parameter per value of thecontrol parameters; judging that, when said shaking amount exceeds anappointed value, the servo control system reaches oscillationcriticality; and adjusting said control parameters by returning thevalues of the control parameter by only appointed steps.
 2. The methodfor detecting oscillation criticality of a servo control systemaccording to claim 1, wherein said shaking amount is a value of astandard deviation of a number of times of inverting a sign of adifference in a value of said control parameter and the value of thecontrol parameter during a previous sampling time during a predeterminednumber of samples.
 3. A method for detecting oscillation criticality ofa servo control system, comprising: stepwise changing value of at leastone control parameter, measuring a shaking amount of the controlparameter, where the shaking amount is an unevenness in a frequencycomponent of oscillations of a torque command per value of the controlparameter; judging that, when said shaking amount exceeds an appointedvalue, the servo control system reaches oscillation criticality; andadjusting said control parameters by returning the values of the controlparameter by only appointed steps.
 4. The method for detectingoscillation criticality of a servo control system according to claim 3,wherein said shaking amount is a value of a standard deviation of anumber of times of inverting a sign of a difference in a value of saidtorque command and the value of the torque command during a previoussampling time during a predetermined number of samples.
 5. A method fordetecting oscillation criticality of a servo control system comprising:measuring a shaking amount of at least one control parameter, where theshaking amount is an unevenness in a frequency component of oscillationsof said control parameter during an operation of a servo control system;judging that, when said shaking amount exceeds an appointed value, theservo control system reaches oscillation criticality; and automaticallyadjusting said control parameter changing values from present setvalues.
 6. The method for detecting oscillation criticality of a servocontrol system according to claim 5, wherein said shaking amount is avalue of a standard deviation of a number of times of inverting a signof a difference in a value of said control parameter and the value ofthe control parameter during a previous sampling time during apredetermined number of samples.
 7. A method for detecting oscillationcriticality of a servo control system comprising: measuring a shakingamount where the shaking amount is an unevenness in a frequencycomponent of oscillations of a torque command during the operation ofthe servo control system; judging that, when said shaking amount exceedsan appointed value, the servo control system reaches oscillationcriticality; and automatically adjusting a control parameter by changingvalues from the present set values.
 8. The method for detectingoscillation criticality of a servo control system according to claim 7,wherein said shaking amount is a value of a standard deviation of anumber of times of inverting a sign of a difference in a value of saidtorque command and the value of the torque command during a previoussampling time during a predetermined number of samples.